I have a bunch of books showing captured U.S. equipment, but the photos are copyrighted are you would need to buy the books to see them...sorry.
Kennethc outlined things well. In addition to those vehicles he mentioned, I've also seen photos of M3 halftracks, Dodge staff cars, an M-10 tank destroyer, and probably a few others.
From everything I've seen, major modifications were seldom made. Quick repainting and/or camo was sometimes added. Additional protection such as track sections were sometimes added to the front and sides of tanks. The U.S. stars were always painted out and were frequently covered over by German crosses. Typically, captured vehicles were only used in the immediate areas where they had been captured (i.e. vehicles from Kasserine Pass were only used on the U.S. front in Tunisia).
Hope this helps,
Robert